The present invention relates to an optical disk apparatus for reading and writing data from/to an optical disk and, more particularly, to an optical disk apparatus for producing a read signal and a servo error signal having different frequency bands from a detection signal of a same photodetector.
Hitherto, in an optical disk apparatus using a rewritable optical disk medium, both of a reproduction signal to demodulate data and aservo error signal which is used for servo control are produced from a return light of a read beam reflected and diffracted by the optical disk by photoelectric conversion. There are a focusing error signal and a tracking error signal in the servo error signal. The tracking error signal is used for counting the number of tracks at the time of seeking. Upon seeking, the value of the track count is subtracted from the number of tracks up to the target track, thereby obtaining the remaining number of tracks. A target speed corresponding to the remaining number of tracks is generated and an optical head is speed controlled (coarse control) in accordance with the order of acceleration, constant speed, and deceleration. When the remaining number of tracks is equal to 0 during the deceleration, the control mode is switched from a speed control to an on-track control (fine control) and the head is led to the track, thereby accurately seeking to the target track.
Generally, the tracking error signal is detected by a push-pull method (far field method). That is, an image of the primary diffraction light generated by the irradiation of a read beam to the track of a preformat shape sandwitched by grooves on the both sides is formed on a 2-split type photodetector. By obtaining a difference between the light reception signals of two light receiving sections, the tracking error signal is formed.
FIG. 1 shows a reproducing circuit of the reproduction signal and the tracking error signal in a conventional optical disk apparatus together with a read optical system. A laser beam from a laser diode 170 is wavefront converted by a collimating lens 172 and is transmitted through a beam splitter 174 which functions as a half mirror and is formed as an image onto the medium surface of an optical disk 130 by an objective lens 176. The return light as a primary diffraction light by the track shape of the medium surface is reflected by the beam splitter 174. A (P) polarization component transmitted through a polarization beam splitter 178 is formed as an image on a 2-split photodetector 182. An (S) polarization component reflected by the polarization beam splitter 178 is formed as an image on a photodetector 180 to detect a reflection intensity.
Light receiving sections 184 and 186 provided for the 2-split photodetector 182 generate detection currents i.sub.1 and i.sub.2 corresponding to the intensity of the return light. The detection currents i.sub.1 and i.sub.2 are amplified by operational amplifiers 220 and 228 which are DC coupled and are converted into the voltage signals. After that, the voltage signals are differentially amplified by an operational amplifier 370. An output voltage of the operational amplifier 370 is a voltage that is proportional to the difference (i.sub.1 -i.sub.2) between the detection currents i.sub.1 and i.sub.2 and is generated as a tracking error signal TES.
On the other hand, a reproduction signal MO is formed by operational amplifiers 208, 364 and 214. That is, detection currents from the light receiving sections 184 and 186 of the 2-split photodetector 182 are added by AC coupling by capacitors 188 and 196 and the added current is amplified by the operational amplifier 364 and is converted to the voltage signal. On the other hand, a detection current i.sub.0 of the photodetector 180 is also AC coupled to the operational amplifier 208 by a capacitor 206 and is converted to a voltage signal by amplifying. Signal voltages from the operational amplifiers 208 and 364 are AC coupled to the operational amplifier 214 by capacitors 210 and 366 and a difference between them is obtained and is generated as a reproduction signal MO. The MO signal is a signal that is proportional to: EQU i.sub.0 -(i.sub.1 +i.sub.2)
Further, as a light intensity signal ID indicative of a light intensity by the preformat shaped concave and convex portions of the track, a signal which is proportional to: EQU i.sub.0 +(i.sub.1 +i.sub.2)
is obtained by an adding circuit (not shown).
Recently, in association with a high-speed seeking operation of the optical disk apparatus, a frequency of the tracking error signal TES is equal to or higher than 500 kHz in correspondence to the maximum speed of the head during the seeking operation, so that a frequency band of at least DC to 500 kHz is required. On the other hand, a frequency band of the reproduction signal MO for demodulation of data lies within a range from 10 kHz to 20 MHz. In this instance, as shown in FIG. 1, when the photodetector to detect the return light is commonly used for both of the reproduction signal and the servo signal, the frequency bands which are necessary by them overlap, so that the signals having the frequency bands necessary for both signals cannot be separated by a simple band separating filter comprising a resistor and a capacitor.
As shown in FIG. 2, the photodetector for the reproduction signal and the photodetector for servo are separately provided and the return light is separated and is independently converted to the electric signal. That is, a part of the return light separated by the beam splitter 174 is reflected by a beam splitter 348 and is formed as an image on the 2-split photodetector 182 only for use in the servo. The (P) polarization component of the remaining return light which has transmitted through the beam splitter 348 is transmitted by a polarization beam splitter 380 and is formed as an image onto a photodetector 382. The (S) polarization component is reflected and is formed as an image onto the photodetector 180.
As for the 2-split photodetector 182 for use in only the servo, the tracking error signal TES of a frequency band of DC to 500 kHz is formed by the operational amplifiers 220, 228, 370 in a manner similar to FIG. 1. On the other hand, with respect to the photodetectors 180 and 382 for reproduction, the signals from those photodetectors are amplified and converted into the voltage signals by the AC coupling by the amplifiers 208 and 386. After that, the voltage signals are differential amplified by an operational amplifier 394, thereby forming the reproduction signal MO having a frequency band of 10 kHz to 20 MHz that is proportional to (i.sub.01 -i.sub.02).
However, in the case where the photodetectors are separately provided for servo and reproduction as shown in FIG. 2, the optical system is complicated, resulting in an obstacle for miniaturization of the optical head. Since the optical parts such as a beam splitter using a prism which functions as a half mirror and the like are super high precision parts on the order of angstrom, they are obstacles in decrease in costs. Further, in recent years, due to an increase in recording density of the optical disk medium, the quality (S/N ratio) of the reproduction signal is becoming significant. In this case, when a quantity of light is decreased by the beam splitter 348 newly added, noises of the photodetectors and amplifiers are dominant, so that the reliability is deteriorated.
Therefore, as shown FIG. 1, it is desirable that the photodetector is commonly used for servo and reproduction without increasing the number of beam splitters. In this case, in order to cope with the realization of a wide band, it is considered that, after the return light was converted into the electric signal, the electric signal is transmitted through an operational amplifier having very wide frequency characteristics of DC to 20 MHz, and after that, the reproduction signal and servo error signal are electrically formed. Namely, as shown in FIG. 3, the detection currents i.sub.1 and i.sub.2 from the 2-split photodetector 182 are first amplified by operational amplifiers 378 and 384 having very wide frequency characteristics of DC to 20 MHz, thereby converting into the voltage signals. The other constructions are substantially the same as those in FIG. 1. As operational amplifiers 378 and 384 for the above processes, high speed operational amplifiers each having a high through rate are necessary, so that the costs remarkably rise. For example, the costs of the high-speed operational amplifier are several times as high as those of the general operational amplifier. There are problems such that an electric power consumption of the high speed operational amplifier is large and a size of package thereof is also large.